The output is the following:
java.lang.NullPointerException
at com.sun.javafx.tk.quantum.EmbeddedScene$PaintRunnable.paintImpl(EmbeddedScene.java:154)
at com.sun.javafx.tk.quantum.EmbeddedScene$PaintRunnable.run(EmbeddedScene.java:230)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:441)
at java.util.concurrent.FutureTask$Sync.innerRunAndReset(FutureTask.java:317)
at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:150)
at com.sun.prism.render.RenderJob.run(RenderJob.java:29)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at com.sun.javafx.tk.quantum.QuantumRenderer$ObservedRunnable.run(QuantumRenderer.java:70)
at java.lang.Thread.run(Thread.java:662)

This happens when I change the x and y of a translation transform associated with a Group while the program is running. I make that change on a mousePressed event.

Sorry but I can't supply a sample app, because my app is too big..